How to use this calculator
- Measure the length and width of the room and the height of the walls, all in feet.
- Enter the number of doors and windows. They are subtracted from the wall area.
- Choose whether to include the ceiling. Ceilings often use a different paint, so you may want to run the calculator twice.
- Enter the number of coats. Two is standard, and a big color change may need more.
- Check the coverage per gallon on the can. It is usually printed as a number of square feet, and 350 is a common figure.
How the numbers are calculated
Wall area is the distance around the room multiplied by the ceiling height. The calculator then subtracts 20 square feet for each door and 15 for each window, which are common allowances.
The remaining area is multiplied by the number of coats and divided by the coverage of one gallon. The result is rounded up to a whole gallon. If you include the ceiling, its area, length times width, is added before multiplying by the coats.
Worked example
A room 12 ft by 14 ft with 8 ft walls has 2 × (12 + 14) × 8 = 416 sq ft of wall. One door and two windows remove 20 + 30 = 50 sq ft, leaving 366 sq ft. Two coats cover 732 sq ft, and at 350 sq ft per gallon that is 2.09 gallons, so buy 3 gallons.
Tips before you buy
- Rough or textured surfaces soak up more paint. Lower the coverage number if the walls are textured or unprimed.
- Changing from a dark color to a light one, or covering new drywall, usually calls for a coat of primer first.
- Buy all the paint you need at once and mix cans together so the color stays the same across the room.
- Keep a little left over for touch-ups, and write the color name and code on the lid.
- Trim, baseboards and closets are not included, so add a quart if you plan to paint them.
Frequently asked questions
How much paint do I need for a 12 by 12 room?
With 8 foot ceilings, one door and two windows, the walls are about 334 square feet. Two coats cover 668 square feet, which is just under two gallons at 350 square feet per gallon.
How much does a gallon of paint cover?
Most labels say about 350 to 400 square feet per gallon on a smooth, primed surface. Rough surfaces use more.
Do I need one coat or two?
Two coats give an even finish and better color for most jobs. A single coat can work when you repaint a similar color over a clean, sound surface.
Should the ceiling use the same paint as the walls?
Usually not. Ceilings are often painted with a flat ceiling paint, so calculate the ceiling separately if you want a different product.
Quick answers
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These results are estimates for planning purposes. Material sizes, densities and local requirements vary, so confirm quantities with your supplier or a qualified contractor before ordering.
Last updated October 11, 2026.